The present invention relates to the production of reinforced laminates such as used in circuit board manufacture and more particularly to the use of a polyetherimide/epoxy therefor.
Metal-clad boards, particularly such boards for use in fabricating printed circuits, are well known in the art. The simplest of such boards generally comprises a resinous plastic substrate to which is bonded at least one thin sheet of an electrically conductive material, preferably copper. The resinous plastic substrate can be clad with the metal foil on one or both sides, depending upon the desired use, and can be rigid or flexible depending upon the composition of the resinous plastic substrate, the choice of reinforcement (if any), and the use to which the board is to be put.
In preparing rigid metal-clad boards, it is common to form individual lamina, commonly called prepregs, by formulating a resinous binder composition made from epoxy, modified styrene, or the like. A solvent solution of the resin is placed in an apparatus known as a "dip tank". Continuous webs of reinforcement can be preimpregnated in the tank and then dried in a vertical or horizontal treating tower or oven. Normally, the resin is partially cured or B-staged after exiting the treater tower or oven. The copper foil, optionally coated with an adhesive, is placed on one side of the prepreg and subjected to heating under pressure to effect a bond between the metal foil and the substrate. Multiple prepregs can be used in forming a single composite board. Additionally, multilayer printed wiring boards will have a number of interposed laminae and copper sheets.
Pressing of the boards can be effected in a press by placing the foil/substrate structure between the platens and closing the press, or a continuous belt can be used. The curing cycle in the press will depend upon nature and thickness of the laminate, the time and temperature of the cycle being those required to cure the substrate and the bonding adhesive layer, if present. Sufficient pressure is required to effect adequate flow of the adhesive and/or substrate resins in order to wet-out and bond adequately. The pressure must be sufficient to prevent blistering which is due to the release of gases resulting either from retained volatiles in the substrate or adhesive layers, or resulting from by-products of the curing process.
Heretofore, poyetherimide sheets have been proposed as substrates to which copper can be clad in the formation of printed circuit boards. Reinforced composite structures also have been made utilizing polyetherimide resins. U.S. Pat. No. 4,297,385 shows the use of polyetherimide resins. U.S. Pat. No. 4,118,535 shows the use of a polyetheramide acid for coating metals. U.S. Pat. No. 4,118,535 shows the use of a polyetheramide acid plus an epoxy for forming coatings for metals. U.S. Pat. No. 3,682,960 shows the use of poly bis(4-aminophenyl) ether pyromellitamide plus a copolymer of trimellitic anhydride and methylene dianiline for forming coatings for metals. U.S. Pat. No. 4,562,119 shows a siloxane-modified polyamide, polyimide, or polyamide-imide curable coating for metals.